1. Field of the Invention
The present invention relates to a distance measuring apparatus for optically accurately measuring the distance to an object to be detected by a triangulation method and, more particularly, to a distance measuring apparatus in which a light amount of the reflected light is held to a predetermined level.
2. Prior Art Statement
Hitherto, for instance, as shown in JP-A-57-44809, a distance measuring apparatus with use of the triangulation method as shown in FIG. 17 has been known as an apparatus to detect the distance to an object. According to such an apparatus, a light emitted from a light projecting device 1 such as a light emitting diode or the like is converted into a parallel light beam 3 by using a collimating lens 2. The light beam 3 is projected in the direction to detect a distance. A condenser lens 5 is provided to detect the reflected light from an object 4 existing at a position which is away from the lens 2 by a certain distance. A position sensitive device (hereinafter, abbreviated to a PSD) 6 to output currents from both ends on the basis of the light detecting positions is arranged behind the condenser lens 5. When the object 4 interrupts the light beam 3 at a position within a predetermined distance range from the front surface of the lens 2, the diffused reflected light from the object 4 are converged by the condenser lens 5 and impinged upon the PSD 6. Therefore, the position of the reflected light which is impinged upon the PSD 6 changes depending on the distance from the lens 2 to the object 4 to be detected. A power source 7 to bias is connected to the PSD 6 and two analog current outputs corresponding to the light impinging positions are generated from both ends of the PSD 6. The output signals are input to I/V (current to voltage) converters 8 and 9, respectively. The I/V converters 8 and 9 convert the current outputs from the PSD 6 into the voltage signals and supply them to a subtracting circuit 10 and an adding circuit 11. A dividing circuit 12 divides an output of the subtracting circuit 10 by an output of the adding circuit 11, thereby outputting the normalized signal corresponding to the light detecting position irrespective of the sum of the light detection amounts. The output signal is input to, e.g., a comparing circuit 13. The comparing circuit 13 detects a signal exceeding a predetermined threshold level and gives a detection signal to a signal processing circuit 14. The light projecting device 1 is lit on through a light projecting device drive circuit 16 by a pulse output of a pulse oscillator 15. The output of the pulse oscillator 15 is also given to the signal processing circuit 14. When a detection signal is input for a pulse oscillating period, the signal processing circuit 14 outputs the detection signal to the outside through an output circuit 17.
According to such a conventional distance measuring apparatus, the position detection signal is obtained by dividing the output of the subtracting circuit by the output of the adding circuit by the dividing circuit. Therefore, there is a drawback such that its error amount largely changes in dependence on the light amount level. That is, although the accurate position detection signal is derived if the light amount level is sufficiently large, if the amount of light which impinges upon the PSD 6 is small, the position detection signal largely changes due to a slight fluctuation of the output difference of the I/V converters 8 and 9. Therefore, there is a drawback such that the measuring accuracy varies by the light amount. If the gains of the I/V converters 8 and 9 to convert the light detection current signals into the voltage signals are raised to increase the input level of the dividing circuit, when an object exists at a close position and the light sensitive level rises, the input level is saturated and the accurate division output cannot be derived. Therefore, there is a problem such that the fluctuation of the input level of the dividing circuit is large and errors easily occur.
On the other hand, since the fluctuation of the input level of the dividing circuit is large, there is a problem such that errors easily occur unless the dividing circuit of a wide dynamic range and a high response speed is used.
Further, in the case of constructing the dividing circuit by using a log amplifier, both inputs are limited to the inputs .having the same sign. Therefore, an object can be detected only in the half region of the PSD 6 such that the difference between the outputs of the I/V converters 8 and 9 is always positive. There is a drawback such that the use efficiency of the PSD 6 is low and its resolution deteriorates.